1. Field of the Invention
This invention relates to methods for determining body composition from the electrical characteristics of the body and, more specifically, to methods for measuring body cell mass, extracellular mass, total exchangeable potassium and total exchangeable sodium from total body electrical resistance and reactance.
2. Description of the Prior Art
A variety of techniques and devices have been devised to measure the various components of body composition such as body fat, lean body mass, body cell mass and extracellular mass in humans and animals as a means of assessing their nutritional status and general health.
Particular interest has been directed to body cell mass which is the potassium rich, energy exchanging component of the body responsible for all metabolic activity and extracellular mass which is the nonliving component of the body composed of water and extracellular solids, such as bone, collagen and adipose, is metabolically inactive, does not consume any oxygen and serves mainly support and transport functions.
Malnutrition is characterized by a loss of body cell mass, accompanied by an expansion of the extracellular mass. When a malnourished state is corrected with the appropriate nutritional support the reverse occurs; the body cell mass increases, while there is an accompanying contraction of the extracellular mass. Body cell mass contains 98-99% of all total body potassium. The intracellular potassium concentration of the body is fairly constant thus resulting in a very close relationship between total body potassium and body cell mass. Further, since the majority of the body's sodium is in the extracellular mass, there is a close relationship between total exchangeable sodium and the total extracellular mass. However, because some of the sodium in bone is fixed within the bone matrix and does not exchange with injected isotopes of sodium, total body sodium and total exchangeable sodium are significantly different. As a result the ratio of total exchangeable sodium divided by the total exchangeable potassium, which is a measure of the extracellular mass expressed as a function of the body cell mass, is a sensitive measure of the nutritional state of a patient.
The measurement of body cell mass and extracellular mass requires specialized and expensive instrumentation or invasive techniques. Body cell mass has been determined by measuring the total body content of 40K, the naturally occurring radioactive isotope of potassium. 40K comprises 0.0118% of all naturally occurring potassium and is measured using expensive total body counter equipment which must be constructed with special steel, and which requires difficult calibration.
Another approach which has been employed to measure the body cell mass is isotope dilution. Two techniques have been established. The first involves injecting 42K, a short lived radioactive isotope of potassium. Twenty four hours following the isotope injection, blood samples are obtained and from the specific activity of 42K, the body cell mass is determined. Because 42K has a very short half life of 12.4 hours, its routine use is extremely difficult, except as a research tool. An alternate technique has been developed to measure both the body cell mass and the extracellular mass. This technique employes the simultaneous intravenous injection of 22Na, a radioactive isotope of sodium, and 3H2O, water labeled with a radioactive isotope of hydrogen. This technique avoids the difficulty associated with short lived radioactive isotopes. However, both techniques suffer from the need to inject radioactive substances, thus exposing the subjects to radiation. With both techniques urine and blood samples must be obtained during the 24 hours following the administration of isotopes. Thus at least 24 hours are required to obtain a single measurement. Furthermore, repeated measurements within a few days are difficult to obtain.
These methods or techniques also suffer from other problems such as:
1. They are invasive, i.e., radioactive isotopes or other toxic substances are injected into the patient for subsequent measurement. PA0 2. The patient is subject to external characteristics, such as renal function defects, which effect the overall measurement of the injected substance. PA0 3. Blood and other in vivo tissues may be required for subsequent analysis. PA0 4. There is a considerable length of time required to obtain the required samples for subsequent data analysis. PA0 5. Repetition of the process may be difficult within a short period of time. PA0 6. Such processes have been expensive in terms of the number of expendable supplies, laboratory personnel, tracer substances and tissue assays utilized.
Thus, it would be desirable to provide a method for measuring total body cell mass and total extracellular mass of a subject which overcomes the above-listed problems. It would also be desirable to provide a method for measuring total body cell mass and total extracellular mass which is simple, non-invasive, inexpensive, accurate and can be repeated often. Finally, it would be desirable to provide a method for measuring total body cell mass and total extracellular mass which is non-invasive, i.e., does not require the injection of potentially toxic or harmful substances into the body or the sampling of either blood or other tissue and fluids.